Migrate docs to antora

- This is basically copy from main branch minus all
  terminal ui things.
- Relates #971

spring-shell-docs/modules/ROOT/pages/basics/index.adoc

@@ -0,0 +1,17 @@
+[[using-shell-basics]]
+= Basics
+:page-section-summary-toc: 1
+
+This section covers the basics of Spring Shell. Before going on to define actual commands and options,
+we need to go through some of the fundamental concepts of Spring Shell.
+
+Essentially, a few things needs to happen before you have a working Spring Shell application:
+
+- Create a Spring Boot application.
+- Define commands and options.
+- Package the application.
+- Run the application, either interactively or non-interactively.
+
+You can get a full working Spring Shell application without defining any user-level commands
+as some basic built-in commands (such as `help` and `history`) are provided.
+

spring-shell-docs/modules/ROOT/pages/basics/reading.adoc

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+[[using-shell-basics-reading]]
+= Reading Docs
+:page-section-summary-toc: 1
+
+Throughout this documentation, we make references to configuring something by using
+annotations or programmatic examples.
+
+NOTE: There are two annotation models, xref:commands/registration/annotation.adoc[annotations]
+referred to new annotation model, xref:commands/registration/legacyannotation.adoc[legacy annotations]
+referred to old legacy annotation model.
+
+Old legacy annotation model mostly relates to use of `@ShellMethod` and `@ShellOption` and
+new annotation model relates to use of `@Command`.
+
+The programmatic model is how things are actually registered, even if you use annotations.
+
+NOTE: Currently whole documentation structure is in transit to provide better
+structure how things can be used using different ways to provide configurations.
+So pardon a for little confusion now and there during a transit.